1. Field of the Invention
The present invention relates to a charge control apparatus and, more particularly, to a charge control apparatus which can appropriately store power generated by a solar cell in a storage battery and protect devices in the apparatus even in maintenance such as exchange of the storage battery.
2. Description of Related Art
In recent years, global warming, exhaustion of fossil fuels, radioactive contamination due to nuclear accidents and radioactive wastes pose problems, and awareness of the terrestrial environment and energy is rapidly growing. Under these circumstances, a solar cell which generates power from solar radiation as an inexhaustible and clean energy source is expected.
Systems using a solar cell have a variety of scales from several W to several thousand kW. The types of systems are also rich in variety, and there are a system which directly uses power generated by a solar cell, a system which stores power in a battery, a system which uses both the photovoltaic power and a commercial power source, and the like. Of these systems, the system which stores power generated by the solar cell in a battery is often connected to a charge control apparatus for controlling charge of the battery such that the battery performance can be maintained for a long period.
FIG. 4 shows an example of such a charge circuit having a charge control function, which is disclosed in Japanese Patent Laid-Open No. 55-127075. Referring to FIG. 4, reference numeral 1 denotes a solar cell panel; 2, a storage battery as a secondary battery; 5, a voltage detector; 7, a reference voltage generator; 9, a comparator; 12, a shunt; and 14, a reverse current preventer. Power generated by the solar cell panel 1 is stored in the storage battery 2. At this time, the comparator 9 is used to compare a signal obtained upon detecting the terminal voltage of the storage battery 2 by the voltage detector 5 with a signal generated by the reference voltage generator 7. If the voltage of the storage battery 2 exceeds a predetermined value, the shunt 12 is operated to short-circuit the output from the solar cell panel 1. However, the charge circuit shown in FIG. 4 has the following problems. The solar cell panel 1 has the voltage-current characteristics as shown in FIG. 5. For this reason, when the storage battery 2 is exchanged or a cable to the storage battery 2 is disconnected in the charge circuit shown in FIG. 4, the open circuit voltage of the solar cell panel 1 is applied to the voltage detector 5. Since this voltage is higher than the above-described predetermined voltage which causes short circuit of the output from the solar cell panel 1, it is determined that the storage battery 2 is overcharged. As a result, the shunt 12 operates on the basis of the output from the comparator 9 to short-circuit the output from the solar cell panel 1. At this time, the output voltage from the solar cell panel 1 abruptly lowers. The comparator 9 determines that the overcharged state is canceled, and the short-circuited state by the shunt 12 is canceled. Alternatively, when power supply is stopped, the closed state of the shunt 12 cannot be maintained, and the short-circuited state by the shunt 12 is canceled. The open circuit voltage is applied to the voltage detector 5 again. The above-described short-circuit operation and short-circuit canceling operation, i.e., the opening and closing operations of the shunt 12 are repeated. When such a kind of oscillation is repeated, constituent components such as a transistor used for the shunt 12 may generate heat and be damaged, as will be described later in detail. In addition, when a relay is used as the shunt 12, the electric contact may be degraded or seized in a short time upon repeating the opening and closing operations.
FIG. 6 shows a charge circuit capable of avoiding the above disadvantages and preventing damage to devices constituting the shunt 12, which is disclosed in Japanese Patent Publication No. 6-67135. Referring to FIG. 6, reference numeral 8 denotes a delay device, and the remaining parts are the same as those in FIG. 4.
As the characteristic feature of the circuit shown in FIG. 6, the delay device 8 is inserted between the differential amplification section as the comparator 9 and the shunt 12 to set a delay time from the operation of the comparator 9 to the operation of the shunt 12. By setting the delay time, the repeating period of opening and closing of the shunt 12 becomes longer than that of the circuit shown in FIG. 4. Loss or heat of the transistor and the like constituting the shunt 12 is decreased, so damage to the transistor and the like can be prevented. In the circuit shown in FIG. 6, however, the number of components of the circuit increases because of addition of the delay device 8.
Causes for the loss and heat will be described next. In the charge circuit shown in FIG. 4 or 6, the comparator 9 and the reference voltage generator 7 often share the power supply of the control circuit, and the power supply of the control circuit often has a capacitor having a relatively large electrostatic capacitance. In this case, even when the output from the solar cell panel 1 is short-circuited, the power supply voltage of the control circuit does not immediately lower, and the reference voltage to be supplied to the comparator 9 does not immediately lower, either. On the other hand, when the shunt 12 using an active element such as a transistor, a control loop is formed through voltage detector 5-comparator 9-shunt 12-reverse current preventer 14-voltage detector 5, thus a regulator so called a shunt regulator is formed. Therefore, a voltage Vs proportional to the voltage Vr to be supplied to the comparator 9 as the reference voltage is supplied across the voltage detector 5 from the solar cell panel 1. However, the reverse current preventer 14 is biased in a reverse direction, for some time, by voltage Vc stored in the capacitor of the control circuit. That is, the above control loop is not formed in a period that the reverse current preventer 14 is biased in the reverse direction, thus a voltage applied across the shunt 12 is to approximately zero volt thereby a transistor as the shunt 12 operates in a switching mode.
Later, the voltage Vc and Vr are declined, and then a relationship Vs&gt;Vc is formed at sometime. Since, the control circuit and the shunt 12 operate as the shunt regulator, thus the voltage supplied across the shunt 12 is locked into Vs.
As shown in FIG. 7, while the shunt 12 is closed, the voltage Vs is applied to the transistor as a switching element of the shunt 12, and at the same time, a current flows to generate a loss. Therefore, the transistor as a switching element generates heat. Note that Voc in FIG. 7 represents the open circuit voltage.
The loss and heat increase as the opening/closing period of the shunt 12 becomes short, as a matter of course. In addition, when the storage battery 2 is connected, the control loop is not formed because of the reverse current preventer 14 which is reverse-biased. Only a low voltage is applied to the transistor of the shunt 12, so neither loss nor heat is generated.
Therefore, in the charge control apparatus shown in FIG. 4 or 6, a sufficient heat dissipation member or structure is required in consideration of not the normal operation but maintenance such as exchange of the storage battery 2 and disconnection. The heat dissipation member or structure increases the cost of the apparatus.